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Datum Transformations

Datum Transformations

A datum transformation is the process of bringing coordinate values referenced to one defined datum into another datum system. In order to perform an exact datum transformation the point's position must be recalculated using the original measurement observations that produced the latitude longitude position.

NTv2 (National Transformation Version 2)

NTv2 (National Transformation version 2) is a grid of accurate datum shifts in a specific format originally devised by the Geodetic Survey of Canada and adopted for use by Australia, France and Germany, etc.

Datum grids are stored in GSB "grid shift" files in the Cadcorp SIS 7.0\NTv2 sub-folder and are described in an XML file (NTv2.xml) that contains one or more Geodetic Datum pairs, referenced by Epsg number, for use with the grids. Cadcorp SIS 7.0 currently supports NTv2 Datum transformations for Canada, Australia, France and Germany.  Users can add additional grid shift files as required and modify the NTv2.xml file with the relevant Geodetic Datum pairs and name of the .gsb file.

Since NTv2 transformations are performed for a specific region, only those Geodetic Datums, the Epsg numbers of which exactly match the Geodetic Datum names specified, will be transformed using NTv2. Other datums will be transformed using the general purpose routines within Cadcorp SIS 7.0.

When Cadcorp SIS 7.0 starts, it will locate the .gsb file and accompanying .xml file and will automatically enable the conversion from one Geodetic Datum to a second Geodetic datum, using the grid file (.gsb) specified in the NTv2.xml file.

You will need to set the datum to match one of the source datum codes for the NTv2 transformation to be triggered, these codes are:

If you open the NTv2.xml file you will see these Source Datum codes, along with the target datum codes:

<?xml version="1.0" encoding="utf-8" ?>

<NTv2>

<NTv2File>

<!-- From North American Datum 1927 (NAD27) to North American Datum 1983 (NAD83) -->

<SourceDatum>6267</SourceDatum>

<TargetDatum>6269</TargetDatum>

<FileName>Canada\NTv2_0.gsb</FileName>

</NTv2File>

<NTv2File>

<!-- From Australian Geodetic Datum 1966 (AGD66) to Geocentric Datum of Australia 1994 (GDA 94) -->

<SourceDatum>6202</SourceDatum>

<TargetDatum>6283</TargetDatum>

<FileName>Australia\A66 National (13.09.01).gsb</FileName>

</NTv2File>

<NTv2File>

<!-- From Australian Geodetic Datum 1984 (AGD84) to Geocentric Datum of Australia 1994 (GDA94) -->

<SourceDatum>6203</SourceDatum>

<TargetDatum>6283</TargetDatum>

<FileName>Australia\National 84 (02.07.01).gsb</FileName>

</NTv2File>

<NTv2File>

<!-- From Nouvelle Triangulation Francaise Greenwich (NTF) to Reseau Geodesique Francais 1993 (RGF93) -->

<SourceDatum>6275</SourceDatum>

<TargetDatum>6171</TargetDatum>

<FileName>France\ntf_r93.gsb</FileName>

</NTv2File>

</NTv2>

Entering the source datum codes

The source datum codes are entered via the Datum field drop-down box in the Geoid tab of the Coordinate Reference System dialog.

However, source datum codes cannot be entered directly and must be selected by choosing the corresponding CRS names available from the drop-down box as follows:

this is "North American Datum 1927 (NAD27)" with the source datum code 6267

 

this is "Australian Geodetic Datum 1966 (AGD66)" with the source datum code 6202

 

this is "Australian Geodetic Datum 1984 (AGD84)" with the source datum code 6203

 

this is "Nouvelle Triangulation Francaise Greenwich (NTF)" with the source datum code 6275

 

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